This invention relates to an electroless composite plating solution which is capable of forming a composite plating film having a smooth surface and good uniformity and keeping its plating properties, such as a co-deposition ratio and a deposition rate, even after a long-term use, thus being very stable. More particularly, the present invention relates to an electroless composite plating solution which has wide utility in various fields including automotive sliding members, driving parts of precision instruments such as cameras and timepieces, molds, metallic masks in printing technologies, household appliances such as irons, and industrial blades and tools. The present invention also relates to an electroless composite plating method using the electroless composite plating solution.
A composite plating technique has been developed as a useful technique since about 1950 and has been used in diverse fields of industries.
In the technique, attention has been paid to surface active agents as an important component capable of imparting wettability, dispersion stability and the capability of co-deposition to composite plating solutions comprising insoluble powdery or fibrous materials having water repellence, e.g., fluorinated graphite and fluoroplastic particles such as tetrafluoroethylene (PTFE). In fact, many proposals have been made on the surface active agents since 1970.
For instance, Japanese Laid-open Patent Application No. Sho.49-26133 proposes an electroless composite plating solution wherein there are used, as an auxiliary agent, cationic surface active agents, nonionic surface active agents, or surface active agents exhibiting cationic properties at a pH of the plating solution (i.e., so-called amphoteric surface active agents).
In Japanese Laid-open Patent Application Nos. Sho.49-5832, Sho.52-56026, Sho.52-56147, Sho.52-130434 and Sho.54-159343, various surface active agents usable for composite plating solutions are disclosed. However, they are mainly used in composite electroplating solutions. The surface active agents proposed therein are fluorine-based cationic surface active agents as a main component and, if required, in combination with fluorine-based nonionic surface active agents.
In Japanese Laid-open Patent Application No. Sho.54-159343, it is indicated that fluorine-based cationic surface active agents are inferior in performance as compared with hydrocarbon-based cationic surface active agents.
Further, Japanese Laid-open Patent Application No. Sho.52-56026 describes co-deposition of polytetrafluoro-ethylene (PTFE) in which a reference is made to detailed combinations and concentrations of individual components (surface active agents). It is also stated that with fluorine-free particles (e.g., those particles of MoS.sub.2, SiC, SiO.sub.2 and the like), plating solutions comprising hydrocarbon-based surface active agents showing both cationic and nonionic properties are effective when being used singly or in combination. In this connection, it is stated that a preferred plating solution should comprise combinations of cationic surface active agents in terms of trimethylalkylammonium salts whose alkyl group has 10 to 20 carbon atoms such as cetyltrimethylammonium bromide, hexadecyltrimethylammonium bromide and the like, and wetting agents such as condensates of ethylene oxide and octyl phenol, nonyl phenol, lauryl phenol or the like which can be commercially available under the trade name of "Triton X-100".
In U.S. Pat. No. 4,997,686, a wide variety of combinations of surface active agents useful in electroless composite plating are described. More particularly, it is stated to use surface active agents mainly composed of nonionic surface active agents in combination with the other various surface active agents including anionic and cationic surface active agents.
Japanese Laid-open Patent Application Nos. Hei.5-163580 and Hei.5-163581 describe PTFE electroless composite plating solutions. In Japanese Laid-open Patent Application No. Hei.5-163580, it is described that a specific type of PTFE particle, which has been improved on the surfaces thereof by a two-stage chemical treatment, is used to provide a plating solution which does not contain any surfactant, thereby ensuring a good appearance and a long life. In Japanese Laid-open Patent Application No. Hei.5-163581, there is described an electroless composite plating solution comprising a water-soluble polyvinylpyridine derivative, which is unlikely to cause foam and has a long life.
According to H. Matsuda et al. (Trans. I. M. F., 1994, 72(2), pp. 55-57), there have been reported the results of their studies on PTFE composite electroless plating solutions comprising hydrocarbon or fluorine-based cationic surface active agents (of five types) and a hydrocarbon-based nonionic surface active agent (polyoxyethylene nonylphenyl ether) in combination. In Trans. I. M. F. 1995, 73(1), pp. 16-18, there have been reported studies on plating solutions which comprise a wider range in type and combination of surface active agents including anionic surface active agents.
In this way, the surface active agents used in composite plating solutions should favorably be cationic, nonionic and amphoteric in nature, and thus, it is known that substantially a whole range of surface active agents including those fluorine-based agents along with hydrocarbon and silicone-based agents can be used in this art.
In recent years, however, electroless Ni--P/PTFE composite plating technique has been extensively employed in the plating industry, with the result that there arises the problem that the currently employed electroless composite plating solutions cannot necessarily satisfy requirements of users.
More particularly, electroless composite plating solutions are significantly shorter in life than electroless plating solutions containing no composite material such as PTFE powder. Furthermore, the electroless composite plating solutions have many other problems that the resultant film has a satin-like or lusterless appearance, thus leading to the likelihood of the surface being roughened and the occurrence of various types of appearance defectives, that the deposition rate is slow, and that the plating solution is apt to decompose. In order to ensure an electroless composite plating solution which is utilized in various fields or applications as a general-purpose plating technique, it is essential to solve these problems.
In order to cope with the above problems, the conventional practice is to use surface active agents singly or in combination and severely control the concentrations of the formulation, or to increase the concentrations of co-existing materials (composite materials such as PTFE, SiC and the like) in the solution to a great excess so as to keep and control the plating solution against its aging. In case of electroless Ni--P plating solutions, however, it is substantially inevitable that phosphates and other inorganic salts, which are accumulated matters by aging, be increased in the solution. Thus, nickel phosphite is liable to be crystallized in the solution. In order to avoid this discrepancy, an effort has been made in which the concentration of a complexing agent is raised gradually.
At any event, the electroless composite plating solution has the problem that if the concentration of phosphates, which are accumulated matters by aging, increases, co-deposition ability and dispersion stability of the composite materials are both lowered abruptly. Thus, it has been expected to solve this problem.
Although it is important that an electroless composite plating solution has good performances concerning a co-deposition ratio, a deposition rate and microscopic or macroscopic uniformity of the resultant film, it is also essential for enhancing commercial values in industrial utilization that the plating solution have a long life and be low in cost and easy in handling, and be stable in performance along with a good appearance even after a long-term use through its life.
In this connection, however, in an electroless composite plating solution containing a hypophosphite as a reducing agent, when surface active agents are added to the solution, there arise many inconveniences caused by the addition of the surface active agents, i.e., problems that the resultant plating film has an unevenness of color tone in a striped pattern formed thereon, no plating portion appears, and an unevenness of co-deposition takes place.
When a surface active agent is reduced in amount in order to avoid the adverse influence by the addition of the surface active agent, the dispersion stability of particles dispersed for co-deposition (i.e., composite material) would lower without obtaining a satisfactory co-deposition ratio. In addition, the appearance of the resultant film is liable to become roughened, and a good appearance cannot be expected, thus leading to a problem that the commercial value of platings would lower.
On the other hand, as electroless composite plating proceeds, side products such as a phosphite formed through aging are accumulated in the solution as a result of the oxidative and reductive reactions. Moreover, although consumed components are supplemented, the solution composition in the plating solution always changes, and the electroless composite plating suffers a great influence of such a varying solution composition. This brings about the problem that the life of the bath inevitably becomes shortened.
Where an electroless composite plating solution is continuedly used with stably forming a good deposit, it is necessary to severely control the plating solutions and plating conditions. However, such a management requires very much labor and causes a lower production efficiency.
Accordingly, for the preparation of an electroless composite plating solution which is easy in handling and is capable of forming a plating film with a high commercial value having a uniform appearance, while taking the long life of the plating solution into account, it is not sufficient to use the surface active agents hitherto employed in this art and to appropriately control the concentrations thereof. Hence, there is a strong demand for further improvements and developments of electroless composite plating solutions.